1. Field of the Invention
The present invention relates to a lamp lighting apparatus which lights a discharge lamp, such as an HID (High Intensity Discharge) lamp, using, for example, a short arc discharge.
2. Description of Related Art
An HID lamp is used as a light source of a projection type picture display device (projection display) which modulates light according to image data using a light modulation device, such as an LCD or DMD, and projects the modulated light on the screen thereof so as to display the image. Among HID lamps, ultra-high pressure mercury lamps have a gap between the electrodes thereof which is the shortest (about 1 mm) one of the gaps of HID lamps. Therefore, they are widely used especially for a projection display which desires a point light source.
In an optical system of a projection display, since one certain point with the highest brightness in the arc discharge of the discharge lamp is transferred onto the screen, a change in the flux of light of the discharge lamp and a change in the shape of the arc discharge have a large influence on the brightness of the image projected onto the screen. Therefore, the stability of the shape of the arc discharge of the discharge lamp and flux of light is stringently demanded.
When the frequency of variations in the brightness of the image projected onto the screen becomes below a certain frequency band, the observer recognizes them as a flicker, and comes to feel displeasure. Thus the screen flickering is an issue in the performance of the display.
A phenomenon in which the shape of the arc discharge varies occurs when an arc spot which is a landing position of the arc on an electrode of the discharge lamp moves continuously on the electrode of the discharge lamp or when the arc spot jumps intermittently on the electrode of the discharge lamp. In order to prevent the occurrence of the flicker resulting from movements of the arc spot, a method of superimposing a current whose peak value is higher than a base current having a value which is specified by rated power on the base current for a fixed time period before the polarity of the lamp current is reversed so as to raise the lamp current and to stabilize the arc discharge has been developed (for example, refer to patent reference 1).
In this specification, the current superimposed on the base current is referred to as “superimposed current pulse”, and this method of stabilizing the arc discharge is referred to as “superimposed current pulse lighting method.” The superimposed current pulse lighting method has a great effect of suppressing flickers. Nowadays, it is therefore adopted standardly to light an ultra-high pressure mercury lamp for use in a projection display.
However, a problem with the use of the superimposed current pulse lighting method is that the life of the discharge lamp is reduced. When the discharge lamp reaches the end of its life, the voltage (lamp voltage) across the electrodes of the lamp at the time when an electric discharge occurs becomes higher than a nominal voltage which is determined from the capability of the lighting circuit. It can be considered that the reason why such a phenomenon occurs is because the superimposed current pulse supplies energy more than necessary to the electrodes of the lamp momentarily, and this results in wear in the electrodes and a gap of a certain length or more between the electrodes.
Light generated by the superimposed current pulse does not contribute to the brightness of the image projected onto the screen. In other words, in an LCD, a DMD, or the like which is a light modulation device for use in a projection display, since it is easy to control the gray-scale of the image by making a flux of light which does not vary with time be incident upon the light modulation device, the light generated by the superimposed current pulse is intercepted while be incident upon the above-mentioned light modulation device at the time when the superimposed current pulse is applied to the lamp. As a result, there is a problem that the appliance of the superimposed current pulse reduces the luminous efficiency of the projection display.
These problems are brought to the fore when the superimposed current pulse is constantly applied to the lamp regardless of whether or not there occurs a flicker in the display. Therefore, an appliance of the superimposed current pulse on an as-needed basis reduces the above-mentioned problem. However, an appliance of the superimposed current pulse on an as-needed basis needs a means for detecting occurrence of a flicker which is an optical phenomenon. For example, a method of detecting, as an electric parameter, a change in the shape of an arc which can be assumed to be the same phenomenon as a flicker, and controlling the change is disclosed by the following patent reference 2.
[Patent reference 1] Derra et al., U.S. Pat. No. 5,608,294
[Patent reference 2] JP,2004-39563,A
A problem with prior art lamp lighting apparatus constructed as mentioned above is that while they can control occurrence of flickers by simply applying a superimposed current pulse to a discharge lamp after detecting occurrence of a flicker without causing a large reduction in the life of the discharge lamp, they cannot apply a superimposed current pulse to the discharge lamp when not detecting occurrence of a flicker, and therefore cannot suppress occurrence of flickers completely.